The Internet facilitates the exchange of information and transactions between users across the globe. This exchange of information enables distribution of content to a variety of users. In some situations, content from multiple different sources can be integrated into a single electronic document to create a composite document (e.g., a host electronic document). For example, a portion of the content included in the electronic document may be selected (or specified) by a publisher of the electronic document. A different portion of content (e.g., digital third-party content) can be provided by a third-party (e.g., an entity that is not a publisher of the electronic document). In some situations, the third-party content is selected for integration with the electronic document after a user has already requested presentation of the electronic document. For example, machine-executable instructions included in the electronic document can be executed by a user device when the electronic document is presented at the user device, and the instructions can enable the user device to contact one or more remote servers to obtain third-party content that will be integrated into the electronic document.